1. Field of the Invention
This invention relates to a surface emitting semiconductor laser (referred to as surface emitting laser hereinafter) used as a light source for optical information processing and optical communication, a light source of image processing devices using light, and a light source of electrophotographic copy machines, and more particularly relates to a surface emitting semiconductor laser whose transverse mode is stable, threshold current is small, and output is high.
2. Description of the Related Art
Recently, surface emitting semiconductor lasers, which are used easily for structuring a two-dimensional array, have attracted attentions in optical communication or optical interconnection technical field. Among surface emitting semiconductor lasers, the vertical cavity surface emitting laser (referred to as VCSEL hereinafter), the basic structure of which is manufactured through an integrated semiconductor process, is now being developed for practical application because it is suitable for fabrication of large scale wafers. In the VCSEL, the threshold current is reduced relatively easily by narrowing the emission area (light emission area), therefore the VCSEL is advantageous in that power consumption is reduced in the field of optical switching devices which require simultaneous parallel driving of many elements.
However, in the surface emitting semiconductor laser, it is difficult to increase the output because of small volume of the active area. Currently, multi-mode optical fiber, which is supplied at low cost, is used for optical communication, however, single mode optical fiber will be used mainly in the future because the number of lines is increased greatly. The single mode optical fiber needs the single mode laser to be used together, and the single mode laser is required to be developed.
However, the surface emitting laser is involved in a trade-off problem that if the transverse mode stability is increased, the threshold current increases to result in poor response characteristics or low output, and the surface emitting semiconductor laser which satisfies all the requirements has not been realized.
For example, the proton injection surface emitting laser having the gain waveguide structure, from which stable transverse mode oscillation is easily obtained, causes the slight effective refractive index difference between the current passage area and the peripheral area of the current passage area due to the thermal lens effect to result in weak optical confinement condition, therefore stable transverse mode is obtained event though the diameter of a non-proton injection area (current passage) is enlarged to approximately 10 .mu.m. However, because of weak optical confinement, limited emission efficiency improvement, and significant heat generation, the threshold current is relatively high and response characteristics are poor under the condition without bias.
To cope with this problem, recently the selective oxidation type surface emitting laser having the refractive index waveguide structure has been developed. The selective oxidation type surface emitting laser has a refractive index waveguide passage formed by selective oxidation of a portion of a semiconductor multilayer reflection film adjacent to an active layer, this structure brings about current narrowing effect and strong optical confinement effect, and the threshold current of sub-milliampere order is easily obtained and prompt response characteristics are obtained. However, because the optical confinement effect is significant, the diameter of an emission area should be reduced to 5 .mu.m or smaller in order to stabilize the transverse mode, therefore it is difficult to increase output.
It is reported that the effective refractive index difference of approximately 0.015 between a light emission area and the peripheral area of the light emission area is effective to realize the surface emitting laser which has a large light emission area, emits high output, and has stable transverse mode.
In the above-mentioned selective oxidation type surface emitting laser, the refractive index difference between a selectively oxidized layer (for example, a layer of oxide of AlAs) and a spacer layer (for example, Al.sub.0.5 Ga.sub.0.5 As layer) is large. For example, in the case of the above-mentioned materials, the refractive index of the former AlAs oxide layer is approximately 1.6 and the refractive index of the latter Al.sub.0.5 Ga.sub.0.5 As layer is approximately 3.35, therefore the refractive index difference between these materials is 1.8. As the result, great optical confinement effect is brought about between the selectively oxidized layer and the spacer layer.
To achieve the above-mentioned condition, an optical confinement layer formed of a material having a refractive index somewhat larger than the one of the selectively oxidized layer (the refractive index larger than the one of the selectively oxidized layer and smaller than the one of the spacer layer) may be provided on the peripheral area of the light emitting area instead of the selectively oxidized layer.
The surface emitting semiconductor laser disclosed in Japanese Published Unexamined Patent Application No. Hei 6-69858 is an example of the surface emitting laser having a structure similar to the structure of the surface emitting laser based on this concept. The disclosed surface emitting laser is described with reference to FIG. 19. A bottom DBR mirror (Distributed Bragg reflector mirror ) 104 composed of laminates of an n-AlAs layer and an n-GaAs layer is formed on an n-GaAs substrate 101, and thereafter a bottom spacer layer 105 composed of an n-Al.sub.0.2 Ga.sub.0.8 As, a bottom barrier layer 106 composed of an undoped GaAs layer, a top barrier layer 108 composed of an undoped GaAs layer, and a top spacer layer 109 composed of a p-Al.sub.0.2 Ga.sub.0.8 As layer are orderly deposited, and additionally an n-InGaP layer with a thickness of 200 nm is formed thereon. The n-InGaP layer is removed from a light emission area using wet-etching to form a current narrowing layer 110 having an aperture 112, which is served as a current passage, then a p-Al.sub.0.2 Ga.sub.0.8 As layer 113 is formed to a sufficient thickness for obtaining the flat surface on the top spacer layer 109 exposed to the aperture 112 and the current narrowing layer 110. The p-Al.sub.0.2 Ga.sub.0.8 As layer 113 functions to adjust the distance between the top and bottom resonators (distance from the top surface of the bottom DBR mirror 104 to the bottom surface of the top DBR mirror 116 described hereinafter) to a triple half-wave length of oscillation wavelength .lambda.. The optical distance from the center of the single quantum well active layer 107 (center plane between the top surface and bottom surface) to the top surface of the bottom DBR mirror 104 is a half of the oscillation wavelength .lambda., the thickness is designed so that the amplitude of the standing wave is a maximum at the center of the single quantum well active layer 107. Next, a top DBR mirror 116 composed of laminates of a p-AlAs layer and a p-GaAs layer is formed, and electrodes (not shown in the drawing) is formed on the top surface 115 and the bottom surface to complete the laser.
As mentioned partially hereinabove, the design concept of this surface emitting laser is presumed as described hereunder.
(1) A current is concentrated to the light emission area and the threshold current is reduced because of the current narrowing layer 110.
(2) The material of the current narrowing layer 110 is n-InGaP, the forbidden band width of the layer is larger than the forbidden bandwidth of p-Al.sub.0.2 Ga.sub.0.8 As which is the composition of the above-mentioned spacer layer 109 and-the conduction type is reversed, the larger forbidden band width functions to narrow the current and also the reversed conduction type functions similarly in the aspect of energy barrier.
(3) The thickness of the current narrowing layer 110 is made as thick as 280 nm in order to prevent tunneling and to enhance the energy barrier effect.
(4) The optical distance between the center of the single quantum well active layer 107 and the top surface of the bottom DBR mirror 104 is a half of the oscillation wavelength .lambda., the antinode of the standing wave is positioned at the center of the single quantum well active layer 107.
(5) Because the refractive index of InGaP which is the material of the current narrowing layer 110 is smaller than the refractive index of Al.sub.0.2 Ga.sub.0.8 As which is the material of the top spacer layer 109, the optical confinement effect is not so significant and this condition complies with the above-mentioned concept.
However, the inventors of the present invention tried to make the surface emitting laser that was disclosed in the Japanese Published Unexamined Patent Application No. Hei 6-69585, and found drawbacks as described hereunder and found that this surface emitting laser was not necessarily excellent.
(1) Because there is the current narrowing layer 110 at the position (the position that is oscillation wavelength .lambda. distant from the single quantum well layer 107) where the light intensity is second strongest following the position of the single quantum well layer 107, the light is scattered around the periphery of the aperture of the current narrowing layer 110, and the light confinement effect is insufficient in the vertical direction of the substrate.
(2) The refractive index of GaInP (the material of the current narrowing layer 110) to the laser with an oscillation wavelength .lambda. of 980 nm is 3.2 and the refractive index of Al.sub.0.2 Ga.sub.0.8 As (the material of the above-mentioned spacer layer 109) is 3.4, and because the difference is not so large, the structure satisfies the above-mentioned refractive index difference condition, but the thickness of 200 nm of the current narrowing layer 110 is not so small in comparison with the thickness of 578 nm of the p-Al.sub.0.2 Ga.sub.0.8 As layer 113, therefore the optical confinement effect in the horizontal direction of the substrate is large, and if the diameter of the aperture 112 (light emission area) is 5 .mu.m or larger, the transverse mode becomes unstable and the mode changes to multi-mode.
(3) Because the p-Al.sub.0.2 Ga.sub.0.8 As layer 113 having the thickness of 578 nm is formed on the area having a 200 nm step gap due to the aperture 112, the surface is not flat, and actually a recess area is formed on the central area. The recess area causes the film thickness distribution of the top DBR mirror 116 formed thereon to result in reduced reflectance of the DBR mirror, and the characteristics of this surface emitting laser is not so good as desired.
As described hereinabove, in the surface emitting laser disclosed in the Japanese Published Unexamined Patent Application No. Hei 6-69585, the layer of the material having a small refractive index is provided on the periphery of the aperture 112 which functions as a light emission area, it seems to satisfy the above-mentioned conditions apparently, however, it cannot achieve the object of the present invention that the transverse mode is stable, threshold current is small, and the area of the light emission area is large for obtaining a large light output.
An example of the selective oxidation type surface emitting semiconductor laser is disclosed in U.S. Pat. No. 5,594,751. FIG. 20(a) shows a cross sectional structure of a surface emitting laser, and FIG. 20(b) is an enlarged view of the vicinity of an active area. This surface emitting laser has a first electrode 212 on a first mirror stack 214 and a second electrode 216 on a semiconductor substrate 218 positioned under an active area 220 and a second mirror stack 226, the active area 220 composed of a usual InGaAs multilayer quantum well is in contact on both sides with interposition of the electrodes 212 and 216. The first mirror stack 214 serves as a waveguide resonator for restricting the electric field and confines the transverse optical mode 232. The axial mode of the electric field is determined by the film thickness of the active area 220 located between reflection planes of respective mirror stacks, a first contact layer 222, and a second contact layer 224. The current 230 is flowed into the central portion of the resonator by an insulation layer 227. Because the thickness of the insulation layer 227 is sufficiently thin, the transverse optical mode 232 is not limited, the thin thickness allows the transverse optical mode to have a large diameter to result in efficient single transverse mode operation. The insulation layer 227 comprises a semiconductor layer which is formed using selective technique such as wet etching or wet oxidation.
Herein the case in which wet oxidation is used is described, and in the above-mentioned selective oxidation type surface emitting semiconductor laser, the refractive index of the insulation layer (selectively oxidized layer) 227 provided on the first contact layer 222 is approximately 1.6 (aluminum oxide), on the other hand, the refractive index of the original semiconductor layer is as high as 3 to 3.6, and the refractive index difference is very large. Therefore the large refractive index difference causes light scattering on the periphery of the aperture, which is an light emission area, to result in loss of light, and the threshold current increases and the oscillation efficiency decreases.
To reduce the effective refractive index in order to solve such problem, B. J. Thibeault et al. proposed a method in which a thin AlAs film (.about.30 nm) is used in his article "Reduced Optical Scattering Loss in Vertical-Cavity Lasers Using a Thin (300 .ANG.) Oxide Aperture" (IEEE Photonics Technology Letters, Vol. 8, No. 5, p 593 to 595, 1996), and E. R. Hegblom et al. propose a method in which the end of the selectively oxidized area is formed in taper shape to form lens like refractive index distribution in his article "Vertical Cavity Lasers with Tapered Aperture for Low Scattering Loss" (Electronics Letters, Vol. 33, No. 10, p 869 to 871, 1997). Therefore, in such a surface emitting semiconductor laser, for example, the method in which the insulation layer (selectively oxidized layer) 227 with a thin thickness (&lt;30 nm) is used requires for the thickness of the insulation layer (selectively oxidized layer) 227 to be controlled at the accuracy of several nm order, therefore in consideration of the current selective oxidation technology level, the evenness of film thickness in the horizontal plane of the substrate is insufficient for wafer scale manufacturing of the surface emitting semiconductor laser, and it is difficult to increase the manufacturing yield. In the experience of the inventors of the present invention, it was confirmed that the thin film thickness of an AlAs layer resulted in significantly reduced oxidation rate, and the unevenness of film thickness significantly affected adversely the controllability of the diameter of an aperture formed by oxidation.
On the other hand, the method in which the end of the selectively oxidized area is formed in taper shape is effective to prevent the optical scattering on the peripheral area of the aperture, but it is required for composition and film thickness of the layer to be inserted to be controlled accurately, the requirement leads to troublesome manufacturing process and poor reproducibility. In particular, it is difficult to manufacture surface emitting semiconductor lasers with reduced quality dispersion in wafer scale.
It is an object of the present invention to provide a surface emitting semiconductor laser which is advantageous in that the transverse mode is stable, the threshold current is small, and the area of the light emission area is large for high light output.
It is another object of the present invention to provide a selective oxidation type surface emitting semiconductor laser in which light scattering on the peripheral area of the light emission area is effectively prevented, the threshold current is small, and the emission efficiency is high while the reproducibility and controllability in manufacturing process are maintained excellent.